Electrostatic writing tube



VELECTROSTATIC WRITING TUBE Filed Feb. 7, 1958 Bywnlx rrofA/a/s.

2,978,607 ELECTROSTATIC WRITING TUBE Howard C. Borden, Atherton, Calif., assigner to' A. B. Dick Company Filed Feb. 7, 195s, ser. No. 113,866 s Claims. (Cl. 315-12) This linvention relates to electrostatic printing tubes and, more particularly, to improvements therein.

In an application for Electrostatic Printing System, by John L. lTregay, led May l0, 1957, Serial No. 658,275, and assigned to a common assignee, there is described an limprovedelectrostatic printing tube which has as its target' arwireV array, consisting of small-diameter, short, closely spaced, parallel and mutually insulated wire seg'- ments which have one of -their ends extending through' which the electron beam impinges charge up toward the cathode potential. When the potential of any wire reaches a critical value with respect to the lexternal plane, a discharge takes place between that wire and the plane.

If the writing medium is present, the discharge occursV through it and results in a spot of charge under the wire onl the dielectric writing material which can be developed out to become visible'. Means are providedfor repetitively sweeping the electron beam across the wires, as well as controllingits intensity so that, as the dielectric writingmedium is moved, charges are deposited thereon which, uponv subsequent development, produce visible characters corresponding to the` ones dictated by the deflection and intensity control signals.

From the above brief description, it will bev appreciated that the writing array consists of a' number of isolated wires which have no' direct current return path to the cathode to complete the circuit, except by the electrical discharge which does the writing. This. has two .disadvantages. First, there is no method of controlling thev electrical dischargegthat is, the discharge will continue until the wire capacitance' is discharged."

Because'A of this, a wire will sometimes continue toA write longer than it should'. Second, it is not possibleto put va direct current bias Voltage on the writing wires. It such a bias voltage could be' put on thev writing wires, it would be' possible to maintain the writing wires just below the writing potential, and thereby decrease theV amount of electron-beam current required to 'drive the wires above writing potential.

Accordingly, an object of this invention is to provide an improved target structure' in an electrostatic writing tube for shortening the decay time of an electrical discharge occurring atl a wire end.

Another' object of` this invention is to provide an arrangement for increasing' the sensitivityof an'electrostatic writing tube by providing a target structure whereby a bias may be applied to the writing wires.

Still another. object of the present invention is to provide a structure for an electrostaticwritingtubeto improve itsV writing performance.

Yet another objectofthe presenti invention is'to' provid novel and-unique target--strucfuefor an electrosta e@writ-i-rigtube.m f

These and other objects of the present invention are static writing tube wherein the` writing' points which are' on the external face of the target are connected to the inside of the tube to the region at which the writing elecployed to connect each writing wire inside' the tube to' tron beam will impinge. A different resistance is ema common electrode. The common electrode is brought out external tothe tube to enable its connection to a biasing potential. The biasing potential has its value established below the value required to obtain a dis charge at the writing wire points.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The. invention itself, bothl as to its organization and. method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

Figure 1 is a cross-sectional view of an electrostatic writing tube, including. a target in accordance with this invention;

Figures 2 and 3 are electrical circuitrepresentations ofv Figures 4 and 5 are plan and side views of a targetv construction in accordance with this invention.

Referring now to Figure 1 of the drawings, there may be seen a cross-sectional view of an electrostatic tube including the improved target structure in accordance with this invention. The electrostatic writing tu-be is a conventional cathode-ray tube, except for its target. Thus, it includes an evacuated envelope 10, which has therein a cathode 12, a control electrode 14,. an anode, or accelerating electrode 16, a focus coil 18, and a deflection coil 20. The cathode 12 is connected to a negative potential in the operating potential supply 212. Signals are applied to the intensity control grid Vfrom a signal source 24. Deflection signals are applied to the deflection coil` `from a deflection signal source 26. By means of a volt'- age-dropping resistor 28, the required more-positive operating potential is applied to the anode 16 and to the focus coilk 18.

The target may include a face plate 30, which is at anode potential. Insulatingly supported within a slot in the face plate are a plurality of wires 32. Each one of these wires is separated from the other by a material such as glass, and is mounted so that one end extends through to the outside face of the tube, which is linished in at fashion so that the dielectric Writing medium 34, such as paper, may be passed extremely close to the wire ends which are arrayed in a column, to receive electro static charges. An external plane, or conductor, 36 is connected to the anodes and extends along the column of wire points, and is spaced therefrom. This plane may be considered a ground plane.

In operation, the cathode-ray beam which emanates from the cathode is deected by the deflection signal source to sweep across the portions of the wire which are inside the tube. Whether or not a wire is energized is determined by the intensity-control signals received from the signal source 24 at the time the electron beam is directed upon it. Thus, if thev deflect-ion signals and the intensity-control signals are being received from a suitable scanning mechanism, such as a facsimile scanner or television camera, and rif the writing paper 34 ismoved at a synchronized vertical scanning speed, a pattern of charges will be deposited on the writing paper, which, when subsequently developed, Yusing wellknown e1ec trostatic developing techniques', will producev a' Visible` image ofthe original copy.

The description given thus farV applies on the prior-art electrostatic writing tubes. the writing array of wires consists of a number of iso- It will -be appreciated-tha't-Y lated wires which have no direct current return to the cathode, except by a means of the electrical discharge which does the writing. In view of the fact that the wires are substantially parallel to one another and insulated from one another, the wire-tolwire capacity is high. Thus, an electrical discharge will continue until it has discharged the wire capacitance. Because of this, a Wire will sometimes continue to write longer than it should. Since the dielectric writing medium is always in motion, this can result -in smeared letters.

In accordance with this invention, an improvement in writing quality is obtained by connecting each writing wire to a common electrode 38 through a high resistance.

This may better be seen schematically in Figure 2, where-V in the writing point 40 of a writing wire 32 is connected by means of a resistor 42 to the common return electrode 38. By keeping the capacitance between each wire and the common electrode sufficiently small, then the R-C time constant of each wire can be small, and the voltage on each wire will closely follow the variations in electron beam current hitting the wire; thus, the voltage decay time dictated by the electrical discharge occurring at the wire end may be shortened by the additional leal:- age path provided by the high resistance.

Another limitation on the prior-art electrostatic writing tubes is that it is not possible to apply a D.C. bias voltage on the writing Wires. If such a bias voltage can be put on the writing wires, it is possible to maintain the writing wires just below writing potential and thereby decrease the amount of electron beam current required to drive the wire above writing potential. As may be seen in Figure 1, all that is necessary is to connect the common electrode 38 external to the tube to a potential source, which here is the dropping resistor 28 across the power supply. This has the etect of enabling a much quicker charging up of a wire towardk discharge potential by the electron beam, and thereby increasesv the sensitivity of the electrostatic writing tube, as well as the speed of operation.'

It -has been found that writing is not inhibited by placing a large resistance in the writing wire between the Writing point and the portion at which the electron beam impinges. As seen in the circuit diagram of Figure 3, this resistance 44 connects the writing point to the area of the writing wire at which the electron beam impinges. This area of the writing wire is thereafter connected by means of the resistor 42 to the common-return electrode 38. Of course, the value of the resistor 42 should not be so low that a current will flow therethrough of sufticient amplitude to prevent the writing point from charging up to the discharge potential.

Figures 4 and 5 show side and end views of a preferred arrangement for making the Writing wires for an improved target in accordance with this invention. On an insulator 46, which, as may -be observed from the end view, has a semicylindrical shape, there are spaced a plurality of resistance wires 50. This is mounted in the face of the tube as shown in Figure 1, so that the writing points extending at one end of the semicylinder are external to the tube, and the remainder of the resistance wire is within the tube. The common ground return electrode 38 is positioned to be in contact with the end of these writing wires 50, which are within the tube. This type of structure may lbereadily fabricated by taking a glass-rod insulator, wrapping the resistance wire in a spiral around the insulator, cementing the Wire to the insulator, and then grinding away one-half to leave the semicylindrical shape shown in the end view of Figure 4. The electron beam impinges the center section of the wire array, as may be seen in Figure 5. t

By resistancewire is meant, for example, Nichrome wire or any other wire made of a material having a resistance which is high when compared to the resistance of a conductor such as copper wire.

Still another method for making the array shown in Figures 4 and 5 would be to lay down a resistance film using well-known techniques in the -form of a writing array on an insulating rod and thereafter contacting one end of the resistance lnrwith the common return elec trode.

In accordance V.with the above, there hasl been shown and described herein a novel, useful, and simple arrangement for improving the operation of an electrostatic writing tube by connecting the writing wire points through a high-resistance to a common return electrode which can be connected externally to the tube to afford an arrangement for biasing the writing Wires.

' I claim:

l. The combination with an electrostatic writing tube of the type having a target including a column of points at a writing surface outside said tube, an electron beam means to elevate the potential of selected ones of said points until a discharge occurs from these points to an adjacent ground plane, of a plurality of writing wires made of a material having a resistance which is high when compared to the resistance of copper wire, each of said wires extending from within said tube to a dierent one of said points, and a common electrode connected to all said writing wires and brought out from within said tube.

2. The combination with an electrostatic writing tube of the type having a target including a columnof points at a writing surface outsidesaid tube, an electron beam means to elevate the potential of selected ones of said points until a discharge occurs from these points to an adjacent ground plane, of a plurality of writing wires made of a material having a resistance which is high when compared to the resistance of copper wire, each of said wires extending from within said tube to a different one of said points, a common electrode connected to all said writing wires at their end portions opposite to said points, and means to apply a bias voltage through said common electrode to all said writing Wires which is less than the point discharge potentials but above that of the adjacent ground plane.

3. The combination with an electrostatic writing tube of the type having a target including a column of wire points at a writing surface outside said tube, an electron beam means to elevate the potential of selected ones of said points until a discharge occurs from these Wire points to a ground plane, of a first andrsecond resistor for each of said wire points, the value of each said first and sec ond resistors being high when compared to the resistance of copper wire, said iirst resistor connecting one of said wire points to the interior poltion of said tube at which said electron beam means impinges, and a common electrode, extending from within to the outside of said tube, each said second resistor being connected between said first resistor and said common electrode within said tube.

4. The combination'with an electrostatic writing tube, of an improved target comprising a plurality of writing wires made of a material having a resistance which is high when compared to the resistance of copper wire, means for insulatingly supporting said writing wires spaced apart with one of their ends extending outside of said tube in a column, a common conductive electrode connecting all the other ends of said writing wires within said tube and extending outside of said tube.

5. In an electrostatic writing tube as recited in claim 4 wherein writing wires comprise a resistive film deposited on said means for insulatingly supporting said writing wires.

References Cited in the file of this patent UNITED STATES PATENTS 

